Hello there...I can understand your concern. You can apply 3 ml of sunscreen to face and neck, 3ml to each arm, 6ml to each leg, 6ml to chest and 6 ml for back. Apply the sunscreen to only sun exposed areas. apply it 30 mins before going to sun. Reason for reapplying it after 3 to 4 hours is its effect is short lived. Hence if you don't reapply there won't be any sun protection. It's the same as not applying at all. When reapplying you need not wash the area and reapply. You can apply it over the previous layer itself.